chore(auth): drop dead UpdateProfileRequest; rename shared struct

The combined profile handler (self-edit PUT /api/auth/profile and
admin PUT /api/auth/profile/:id) binds one struct for both paths —
and it's the admin-named one. Its twin UpdateProfileRequest had zero
references left. Delete it and rename AdminUpdateUserRequest to
ProfileUpdateRequest so the name matches who actually binds it; the
comments describing field semantics now stand on their own instead of
pointing at the deleted twin. Wire format (json/form tags) unchanged;
docs schema registration follows the rename.

internal/docs/schema.go

@@ -705,7 +705,7 @@ var Schemas = map[string]Schema{
 	},
 	"PUT /api/auth/profile": {
 		// Shared with admin variant (PUT /auth/profile/:id).
-		Request:  handlers.AdminUpdateUserRequest{},
+		Request:  handlers.ProfileUpdateRequest{},
 		Response: json.RawMessage(`{"message": ""}`),
 	},
 	"DELETE /api/auth/profile": {